Precise gap, thickness, and flatness measurings can be carried out by means of interferometric methods as these methods are not only non-destructive, relatively simple and fast, but also have the highest resolution. Recently, however, it has become evident that for many purposes, particularly the development and production of integrated semiconductor circuits, the resolution of these methods was insufficient since the values to be measured are much smaller than half the wavelength of visible light. The photoresist layers, as an example, which in the production of integrated semiconductor circuits are to be applied in numerous successive process steps, generally have a thickness of approximately 0.5 .mu.m to 1.5 .mu.m. For various reasons, particularly in the production of integrated semiconductor circuits in the submicron range, it becomes increasingly necessary to measure these thicknesses with a precision of at least .+-.10%. As the resolution of all interferometric methods is generally limited by half the wavelength of the light used, there are difficulties in the control of these parameters during the mass production of integrated circuits. Many special methods have been developed and proposed, for example, multi-color interferometry and comparator processes, some of them considerably increasing the resolution of the interference methods. However, these methods involved a remarkable amount of apparatus and they are so complicated, slow and subject to errors that they could be used only in a relatively small number of cases and under specific conditions. Their use in the monitoring and control of large industrial productions is therefore practically excluded in almost all cases.
The co-pending U.S. Application Ser. No. 820,985 filed Aug. 1, 1977, now U.S. Pat. No. 4,188,124 describes an interferometric method with .lambda./4 resolution, where a measuring beam is directed in a predetermined small angle onto a transmission grating, which is parallel to the surface to be measured, that one order of diffraction is directly reflection-diffracted at the grating, and the other three orders of diffraction are reflected from the surface to be measured and transmission-diffracted by the grating to extend in the direction of observation, and by their super-position generate an interference fringe field where the distances between the interference fringes correspond to distances of a quarter wavelength of the radiation used in the object plane. As the grating used has to have reflection and transmission properties adapted in very precise relationships to each other and to the reflectivity of the surface to be measured, and a grating constant adapted to the direction of incidence of the measuring beam and to the direction of observation, and as furthermore the grating has to be arranged very close to maximum 1 .mu.m and exactly in parallel to the surface to be measured, the possible uses for this method are limited. It is in particular not suitable for the monitoring and control of the production of integrated semiconductor circuits as in the monitoring of large numbers of semiconductor chips, due to the unavoidable vibrations and contamination of the optics of the measuring device, measuring errors cannot be excluded. In particular, contamination or soiling of the highly sensitive grating, by the semiconductor chips passing it at a small distance and with high speed, is practically unavoidable, the use of this method for production monitoring and control is bound to create problems which practically exclude its use.